The invention disclosed herein relates generally to dual slope analog to digital converters and control methods therefor, and more particularly to a dual slope analog to digital converter method and apparatus providing for reset of the apparatus in the event an over range input signal condition is encountered.
Dual slope analog to digital converters are well known in which an input signal of unknown value is integrated for a predetermined interval of time to produce an output signal which commences at a predetermined value and changes at a rate depending on the difference in values of the unknown input signal and a fixed reference signal. Thereafter, a signal of known value is integrated to produce an output signal having as a starting value the final value of the signal resulting from integration of the unknown input signal, the integration continuing for a period of time sufficient to return the output signal to its predetermined value. Accordingly, the time interval required for integration of the known signal is indicative of the value of the unknown input signal.
The integrator must be returned to its initial condition in order to commence a suceeding analog to digital conversion. Most commonly, the integrator is returned to its initial condition by completion of the previous conversion process, at which time the output signal of an integrator in the converter is at the proper value for commencing a succeeding integration.
In such operation, a problem may arise where an out-of-range input signal is encountered. In such a situation integration of the unknown input signal results in an output signal of maximum slope in one sense and a corresponding maximum value of the output signal at the end of the known integration interval. Since subsequent integration of the known signal results in an integrator output signal of fixed slope of the opposite sense, substantial time may be taken before the output signal is returned to its predetermined value.
The time required to reestablish initial conditions of the converter after encountering an out-of-range input signal may be several times the length of the interval required to perform an analog to digital conversion of an unknown input signal within the normal range of values. The delay introduced by attempting to process the faulty signal is undesirable, and may unacceptably impede speed of operation of apparatus in which the converter is used.
The applicants have devised a unique and simple method and apparatus for minimizing the unproductive time spent by a dual slope analog to digital converter in attempting to convert an out-of-range signal by immediately returning the converter to its initial state when an out-of-range input signal condition is detected.